1. Field of the Invention
The present invention relates to a circuit arrangement for reducing the decay and start-up transient times of oscillating circuits.
2. Description of the Related Technology
The excitation voltage is switched on and off quickly in oscillating circuits working in intermittent operation. As well as these two stable states, transient states also occur during which the amplitude of the oscillation decays or is built up again. The faster the switching processes take place in a given time period, the greater the proportion of the times during which the oscillation decays or starts again compared to the times in which the oscillating circuit is in a stable state. In order to calculate the proportion of time taken by the transient states:
U=Umaxxc2x7exp(nxc2x7p/Q)xe2x80x83xe2x80x83(1)
provides an exponential description for the decay behavior of the voltage in the oscillating circuit, in which n is the number of periods and a the quality of the oscillating circuit. From the assumption that the oscillation has decayed when the amplitude has fallen to 10% of the maximum value, equation (2) gives at least 10 periods as the number of periods required for this with a quality of Q=14:
n=Q/pxc2x7|in(U/Umax)|=14/pxc2x7|in(10% /100%)|=10xe2x80x83xe2x80x83(2)
Furthermore, equation (2) shows that the number of periods required also increases linearly as the quality increases. An oscillating circuit frequency of, for example, 100 kHz gives a decay time of 100 xcexcs. This doubles to 200 xcexcs when the starting time is taken into account, provided that the starting time is based on approximately the same functional relationship used for calculating the decay time.
Transmitting and receiving units used for transferring data are an important area of application for systems with oscillating circuits working in intermittent operation. For this purpose the oscillating circuit is switched on and off in the transmission units. All the decay and starting times in the oscillating circuit reduce the data transmission rate because the transmission gaps are increased. In the case of digital data transmission at a frequency of, for example, several hundred kHz, the amount of time in the stable state is of the same order as the amount of time in the transient states. A high data transmission rate is very important for use in the field of contactless identification, particularly in the automotive field for the authentication process of transponder and base unit, because the maximum period of time available for the entire process is only 150 ms. If the transponder is supplied with energy from the base unit by inductive means, the transient states of the oscillating circuits lengthen the times during which the transponder cannot pick up any energy from the field of the base unit
Processes which function according to the previous state of the art are known, for example, from the data book of TEMIC Semiconductor GmbH, 1999, p. 404, FIG. 4. The circuit arrangement shown for implementing the process is used in the field of contactless identification systems for triggering oscillating circuit coils in the base units in order to transfer the data and energy to the transponders. According to page 338 of the data book, at an oscillating circuit frequency of 130 kHz the transmission gaps of the base unit lie between 160 xcexcs and 400 xcexcs, depending upon the quality of the oscillating circuit used. The duration of the authentication process between transponder and base unit is at least 50 ms, and this increases up to 120 ms as the quality of the transmission circuits used increases.
The disadvantage of the previous processes for driving the oscillating circuits is that the transient states reduce the data transmission rates of oscillating circuits working in intermittent operation. This results in increased times for the authentication process in the field of contactless identification systems. If the transponder is supplied with energy from the base unit by inductive means, the energy transfer to the transponder is reduced by the longer transmission gaps. The dance between the base unit and transponder, which lies in the range of a few cm, is thus reduced.
An object of the present invention is to specify a process with which the build-up and decay times in oscillating circuits can be significantly reduced in order to increase the data transmission rate between transmission and receiving units. A further object of the invention is to specify a circuit arrangement for implementing the process which can be manufactured easily and economically.
The above objects have been achieved according to the invention in a circuit arrangement for reducing the decay and build-up transient times in an oscillating circuit that operates intermittently and that has at least one capacitor, at least one coil, a control unit which excites the oscillating circuit, at least one switching element, and a current source or a voltage source. The particular features of the invention will be described next.
According to this, the essence of the invention is that, when the excitation voltage of an oscillating circuit is interrupted, the amplitudes of one of the physical quantities which characterize the oscillation are held at their maximum values until the excitation voltage of the oscillating circuit is switched on again. This means that the oscillating circuit can continue oscillating at the maximum amplitude of the quantity which has been maintained without any delay. The applicants investigations have shown that the current or the voltage can be used as characteristic quantities and that one of these quantities can be held both in parallel and serial oscillating circuits it is particularly advantageous if the oscillating circuits are excited with their resonant frequency.
The operation of one embodiment of a circuit arrangement according to the invention will first be explained for the case in which the current is held at its maximum value. When the excitation voltage is interrupted by a switching element driven by a control unit, which excites the oscillating circuit, a current source is linked to the coil of the oscillating circuit which holds the current through the coil at its maximum value. When the excitation voltage is switched on again by the control unit by means of the switching element, the link between the current source and the oscillating circuit is separated again, and the oscillating circuit can continue oscillating at its maximum amplitude without delay. This arrangement is particularly advantageous because the blocking property of the capacitor means that the oscillating circuit does not need to be separated to connect the current source.
A further embodiment of a circuit arrangement according to the invention can be implemented for holding the voltage. To do this, when the excitation voltage is interrupted by a first switching element, a control unit, which excites the oscillating circuit, separates the link between coil and capacitor, and links a voltage source to the capacitor by means of a second switching element in order to hold the voltage at the capacitor at its maximum value.
When the excitation voltage is switched on again, the control unit remakes the link between the coil and the capacitor by means of the first switching element, and separates the voltage source from the oscillating circuit by means of the second switching element so that the oscillating circuit can immediately continue oscillating at its maximum amplitude.